1. Field of the Invention
This invention relates to a polyamide-imide resin insulating varnish and, in particular, to a polyamide-imide resin insulating varnish for providing a covering with a high partial discharge inception voltage, as well as an insulated wire using the polyamide-imide resin insulating varnish.
2. Description of the Related Art
In recent years, hybrid vehicles begin to spread on the background of energy savings. A motor used therefor is inverter-driven so that downsizing, lightening, high heat resistance and high-voltage driving thereof have been accelerated.
As an enameled wire used for a motor coil, a polyamide-imide enameled wire is essential which has all of the excellent heat resistance, mechanical performance endurable in severe coil shaping, and transmission oil resistance, so as to meet the motor performance requirements for downsizing, lightening, high heat resistance. Here, the insulation retention property of the transmission oil resistance may be affected by type or amount of oil additives. However, except for the affection of the oil additives, the transmission oil resistance can be directly affected by the hydrolysis resistance due to water included.
A polyamide-imide resin insulating varnish used for a covering of a polyamide-imide enameled wire is a heat resistant polymer resin that exhibits excellent properties such as heat resistance, mechanical performance, hydrolysis resistance etc. It is generally produced such that two components of 4,4′-diphenylmethanediisocyanate (MDI) and trimellitic anhydride (TMA) are mainly reacted by decarboxylation reaction to be nearly equal in proportion of amide group and imide group in a polar solvent such as N-methyl-2-pryrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), dimethylimidazolidinone (DMI) etc.
As production methods for polyamide-imide resin insulating varnish, isocyanate method, acid chloride method etc. are known. The isocyanate method is generally used in view of its productivity. For polyamide-imide resin, it is well known to use the synthesis reaction of two components, 4,4′-diphenylmethanediisocyanate (MDI) and trimellitic anhydride (TMA) as an acid component.
Also, a polyamide-imide resin may be synthesized such that BAPP and TMA are first reacted under acid excess in proportion of 50/100 to 80/100 so as to improve the polyamide-imide resin property, and MDI is then used to produce the polyamide-imide resin (see JP-B-2897186).
On the other hand, according as the motor is driven at higher voltage, due to superposition of inverter surge, a risk of partial discharge increases so that it becomes difficult to address the inverter surge insulation. One of the drawbacks of the covering of polyamide-imide resin insulating varnish is its high permittivity, and the existence of amide group and imide group can be most intimately related to an increase in permittivity in aspect of the resin structure. As compared to the other enamel resin insulating varnish such as polyester and polyesterimide, the polyamide-imide resin insulating varnish is high in permittivity and low in partial discharge inception voltage while it is significantly excellent in heat resistance, mechanical performance, hydrolysis resistance, oil resistance. Meanwhile, polyimide resin insulating varnish is high in heat resistance, but low in abrasion resistance and hydrolysis resistance. Further, it is inferior to the polyamide-imide in mechanical performance such as coil shaping workability and transmission oil resistance.
In insulated wires, especially, enameled wires used for a motor coil, since the motor is frequently inverter driven for high efficiency, many cases occur where partial discharge is caused by excessive voltage (i.e., inverter surge) and results in insulation breakdown. Recently, according as the motor drive voltage increases, the risk of partial discharge increases further.
Thus, if the polyamide-imide resin insulating varnish could have a low permittivity, an enameled wire excellent in partial discharge resistance can be realized such that it can address the high voltage driving of the motor.
As a technique for enhancing the voltage application life against the partial discharge, a partial discharge resistant enameled wire is disclosed that is produced by coating a partial discharge resistant resin varnish, in which organosilica sol is dispersed in a resin solution, on a conductor (e.g., JP-A-2006-302835 and JP-B-2897186).
In such a partial discharge resistant resin varnish with the organosilica sol dispersed in the resin solution, the solubility between the organosilica sol and the resin solution can contribute to enhancement in partial discharge resistance, and it is proved that the solubility between the organosilica sol and the resin solution composed of polyamide-imide resin varnish etc. can be enhanced by copolymerizing several monomers therewith (see JP-B-3496636).
Another technique is known that electrical field between wires (electrical field applied to air layer existing between the wires) is reduced to prevent the partial discharge to improve the voltage application life.
The above technique includes to reduce the electric field by providing conductivity or semi-conductivity with the surface of the enameled wire, and to electric field by lowering the permittivity of the insulated film.
In the technique of providing conductivity or semi-conductivity with the surface of the enameled wire, the film may be scratched upon the coil shaping to lower the insulation characteristic and its end portion is needed to be insulated. Thus, this technique has many problems and is therefore not good in utility.
On the other hand, in the technique of lowering the permittivity of the insulated film, negative effects are typically caused by the lowered permittivity on the heat resistance and mechanical performance since the lowered permittivity depends on the resin structure. Thus, it is difficult for the above techniques to make a substantial improvement.
The polyamide-imide resin insulating varnish produced by the method of JP-A-2004-204187 is problematic since it is low in softening resistant temperature when it is used as a covering of an enameled wire.
In other words, if the softening resistant temperature is low, the short-circuit risk may increase when it is subjected to high temperature caused by the instantaneous overload of the motor.